Need the scripts of processing the to-be

[Anon]

I need some help about example scripts on:
Question 1:
How to process the confirmation email by clicking the url given in the email
and by replying the email.

Question 2:
I know if using mail(), the "From" field of the mail sent out is always the
web host server's mail sendout server,
not from my own email box. How to send out the email which looks like
sending out from my own email box.

Question 3:
How to let MYSQL automatically delete the unactivated signups?

Thanks!

[Anon]

Answer 1:
have a link in the email, ie: http://www.monkey.com/register.php?secretCode=4G5X75456.

make register.php do the necessary stuff.

Answer 2:
mail("[e-mail]",
"Hello there...",
"bla bla bla...",
"FROM: [e-mail]");

Answer 3:
when a user signs up. set a date to that. and if the user hasn't activated her account in 2 days, it's gets deleted... like this:

// registration
$time = time() + 172800
mysql_query("INSERT INTO userProperties (username, password, registrationTime)
VALUES
('$username','$password','$time')");

you get the rest... or else, don't hesitate to emal me...

[Anon]

Thank you for your advice!
Can I use system's sendmail to send out the emails which look like sent from my email box?